Updates to the Winter Storm Severity Index for 2019/20

The Winter Storm Severity Index (WSSI) is a tool designed to assist National Weather Service (NWS) operational forecasters in maintaining situational awareness of the possible significance of winter weather related impacts based upon the current official forecast. It is also intended to enhance communication to external partners, media and the general public of the expected severity (potential societal impacts) and its spatial distribution due to winter weather. The Winter Storm Severity Index is achieved through the use of Geographic Information Systems (GIS) by screening the official NWS gridded forecasts from the National Digital Forecast Database (NDFD) for winter weather elements and combining those datasets with non-meteorological or static information datasets (e.g., climatology, land-use, population) to create a graphical depiction of anticipated overall impacts to society due to winter weather. The WSSI currently consists of an overall impact graphic and six components including: snow amount, snow load, blowing snow, ground blizzard, snow load and ice accumulation. The overall impact graphic is a composite of the maximum impact from any of the six components.

This presentation will focus on updates to the WSSI as it becomes an experimental NWS product across the CONUS for the 2019/2020 winter season. Topics will include: an updates to the flash freeze algorithm, social science enhancements and 2018/2019 verification. Consistent over alerting led to the flash freeze algorithm change. Examples will be presented on why the over alerting was occurring and what changes will be implemented to decrease the flash freeze algorithm from over alerting. Social science enhancements, including the new color scale, revamped impact definitions and features of the redesigned WSSI web interface will be presented. Finally, a summary on verification of the 2018/2019 season based upon local storm report/ storm data based will be provided.